Increased soil salinity has no effect on the host preference and suitability of the Tamarix biological control agent, Diorhabda carinulata

Abstract

ABSTRACT Tamarix species are major terrestrial riparian invaders in South Africa. The biocontrol agent, Diorhabda carinulata (Coleoptera: Chrysomelidae), was tested as the initial candidate agent in South Africa due to its success against invasive Tamarix in North America. The ability of Tamarix to grow across a broad soil salinity gradient may affect its suitability as a host for D. carinulata, and influence its invasive capability. Therefore, the effect of increased soil salinity was considered a potential influential factor on the host preference of D. carinulata and the suitability of the host plant for the beetle. Choice tests were conducted by exposing beetles to salt-treated and untreated plants of both the invasive alien T. chinensis x T. ramosissima hybrid and indigenous T. usneoides. Leaf chlorophyll content and relative water content were used as a proxy to measure how increased soil salinity affected the health of the Tamarix taxa. Adult host selection and oviposition, and subsequent larval survival and development of D. carinulata on salt-treated and untreated plants were not different on either the invasive hybrid or T. usneoides. Under salt stress, T. usneoides exhibited a greater reduction only in chlorophyll content compared to the invasive hybrid. Increased soil salinity thus does not alter the host preference of D. carinulata, or the suitability of the Tamarix taxa as host plants. The invasive alien Tamarix taxa may be less susceptible to salt-induced stress than T. usneoides, which could be contributing to its invasiveness in South Africa.